Just How Secure Is Airline Security?

No, you haven't stumbled into a gourmet discussion. Bear with me, because there's a very good reason for creating this picture in your mind.

Swiss cheese is noted for its holes, and as much as 30 percent of the surface area of each slice may be taken up by those holes. Just as any security measure we take in aviation has some "holes" in it. Not a 30 percent failure rate (we hope), but some flaws and some ability to fail.

Now, imagine airline passenger screening before 9/11 as a single slice of hole-riddled Swiss cheese, and imagine a type of bullet that cannot penetrate the cheese unless it flies through one of the holes. If that single slice of Swiss cheese security is all you've got standing between the bullet (which represents a terrorist), and seizing control of an airplane or destroying it (which is the bullet's target), as much as 30 percent of the time the bad guys will win.

That's where we were before 9/11.

Adding More Layers

But what if you add more slices of our imperfect Swiss cheese between the bullet and the target? If each new slice represents a new method of denying a terrorist access, then slice by slice the chances of getting all those holes to line up goes down. Soon -- with enough new slices -- you're close to 99 percent secure, even though each and every Swiss cheese security measure remains flawed and has holes!

This is where we are today with airline security against another 9/11-type attack.

We've added layer after layer of new defensive measures in the airport and the aircraft and on the ramp to prevent a terrorist from ever taking control of a commercial airplane again, and the efforts have been highly effective. (The Swiss Cheese model, by the way, comes from the work of Dr. James Reason of Manchester, England, and is also used in strengthening medical safety against patient injuries.)

So what's changed? Well, first, we got rid of what was clearly a sham of a passenger screening system before 9/11 and replaced it with a reasonably disciplined, standardized, courteous force of federal workers under the Transportation Security Administration. That gave a new, imperfect, but vastly improved slice of security cheese.

Second, we massively reinforced all the cockpit doors on commercial airlines in the United States, making it impossible for anyone to just kick through them. Another new slice.

Third, we started paying official attention to who's flying, with additional identification requirements and computer-based systems to identify potentially risky passengers.

While this one, as yet, is anything but foolproof (and we continue to be plagued with politically correct limitations on who TSA can question and when), the system improves constantly, and thus another effective slice of our nearly impervious cheese stands between the would-be terrorist and his target.

Fourth, before 9/11 all pilots and flight attendants were trained to try to comply with a hijacker's demands and just wear him down with slow cooperation. Now, however, we know what the price of passive resistance is, and it's a price we can't afford.

Aircrews -- both pilots and flight attendants -- have an entirely different approach based on careful and aggressive training, and pilots can no longer be blackmailed by threats from the back into opening the cockpit door. Hence, another slice with minimal holes.

Behind-the-Scenes Measures

And there are many more behind-the-scenes, highly effective measures including the basic refusal of passengers to ever sit and tolerate a takeover attempt again.

There is also one highly effective last-resort method ordered by Congress, which the administration has so far thwarted: having pilots licensed to carry guns in the cockpit, a very safe and effective additional slice being withheld from the safety system.

Put this all together in accordance with Reason's Swiss cheese model -- all those new slices between the bullet and the target -- and a terrorist has no practical hope of succeeding.

And that in itself is a major deterrent factor, since in 9/11 one of the hallmarks of the enemy's planning was to make certain that they would succeed. That same potential simply does not exist today.

This work is the opinion of the columnist and in no way reflects the opinion of ABC News.

John J. Nance, ABC News' aviation analyst, is a veteran 13,000-flight-hour airline captain, a former U.S. Air Force pilot and a lieutenant colonel in the Air Force Reserves. He is also a New York Times best-selling author of 17 books, a licensed attorney, a professional speaker, and a founding board member of the National Patient Safety Foundation. A native Texan, he now lives in Tacoma, Wash.